User Interface

Solubility Equilibrium
Pond Chemistry
Select a salt. This presets Worksheet values to a particular example, including the usual value of Ksp for this reaction.

NaCl ⇋ Na+ + Cl-

The current reaction—in this case, the equilibrium between precipitated table salt and its dissolved sodium and chloride ions.
Start over. Clear all values to 0, except for Ksp. To reset Ksp to its original value for the current salt, select another Scenario, then select the current one again.
NaCl mol Moles of the salt to add to the pond (per liter of pond). The convention is to call this moles rather than moles/liter, to stress that the value isn't a concentration. It's a rate of application.

Enter (or edit) the value here, to add more or less salt to the pond.

To specify scientific notation values, use for example “1e-9”. All Scenarios except NaCl use values in scientific notation.

[OH-] mol/liter Ion concentration, of OH- in this example. In the “Input” column (left input), enter the initial value of this ion concentration before adding salt and equilibrating the system.

There's also an input field and a lock button in the “Final” column (right input). If unlocked, this right hand value will simply show the resulting ion concentration after the system runs to equilibrium.

Ion concentration is unlocked. When an ion is unlocked, and salt is precipitated, some of the ion will be consumed in making precipitate. But in certain cases (e.g., OH-, CO32-) this may not be true, because concentrations are controlled by other equilibria (e.g., acid-base). In those cases, you'd enter the fixed concentration value (e.g., based on the water's pH), and lock the ion concentration.

Click the button to lock ion concentration.

Ion concentration is locked. Final concentration not allowed to differ from input value.

Click the button to unlock ion concentration.

Run Button. Apply the inputs to the pond, and run the system to equilibrium.

This means that the initial ion concentrations in the pond are set as per the inputs. Then the salt quantity is sprinkled in and dissolved. Then successive steps are performed, precipitating salt as needed, until Ω <= 1. (Or until a timeout max rounds is reached, in which case a caveat shows at the bottom of the Worksheet.)

Shows an animation on the pond picture. After the animation, final values are shown on the Worksheet.

Ksp The solubility constant of the current salt, at around 25°C.
Q Q is the extent to which the solid salt has dissolved at any point in time, expressed as the product of the concentrations of the two ions (or more than two if relevant).

In the generic salt reaction AB ⇋ A+ + B-, with [A+] meaning the concentration (in M) of ion A+,

Q = [A+][B-]

If AB3 ⇋ A3+ + 3 B-,

Q = [A+][B-]3

In practice, calculating Q is the first step in calculating Ω (Omega).

Ω Ω (Omega) is the ratio of current concentrations to equilibrium concentrations:

Ω = Q / Ksp

If Q < Ksp (and Ω < 1), the solution is “under-saturated” and the solid will tend to dissolve more.

If Q > Ksp (and Ω > 1), the solution is “super-saturated” and the solid will tend to precipitate out.

Pond Animation The pond animation goes through several stages. There is no user interaction. User input is provided on the Worksheet.

First, 15 labels are assigned to salts and ions, weighted by the relative amounts of the inputs. These labels are splashed into the pond.

Some added salts precipitate and sink to the bottom. Some may also dissolve into ions.

Ions meet up with each other and precipitate as salts, if the calculation says so.

Extra salts and ions may appear or disappear as needed to keep the visible ions and precipitate more or less proportional to the calculated quantities.

Smaller labels represent ions or salts that are present, but not in large enough quantities to be shown in proportion to the bigger labels.


by Ginger Booth for Shimon Anisfeld, Copyright © 2014 Yale University